@article{BorgJorgensenFrikeletal., author = {Borg, Leise and J{\o}rgensen, Jakob Sauer and Frikel, J{\"u}rgen and Sporring, Jon}, title = {Reduction of variable-truncation artifacts from beam occlusion during in situ x-ray tomography}, series = {Measurement Science and Technology}, volume = {28}, journal = {Measurement Science and Technology}, number = {12}, publisher = {IOP Publishing}, doi = {10.1088/1361-6501/aa8c27}, abstract = {Many in situ x-ray tomography studies require experimental rigs which may partially occlude the beam and cause parts of the projection data to be missing. In a study of fluid flow in porous chalk using a percolation cell with four metal bars drastic streak artifacts arise in the filtered backprojection (FBP) reconstruction at certain orientations. Projections with non-trivial variable truncation caused by the metal bars are the source of these variable-truncation artifacts. To understand the artifacts a mathematical model of variable-truncation data as a function of metal bar radius and distance to sample is derived and verified numerically and with experimental data. The model accurately describes the arising variable-truncation artifacts across simulated variations of the experimental setup. Three variable-truncation artifact-reduction methods are proposed, all aimed at addressing sinogram discontinuities that are shown to be the source of the streaks. The 'reduction to limited angle' (RLA) method simply keeps only non-truncated projections; the 'detector-directed smoothing' (DDS) method smooths the discontinuities; while the 'reflexive boundary condition' (RBC) method enforces a zero derivative at the discontinuities. Experimental results using both simulated and real data show that the proposed methods effectively reduce variable- truncation artifacts. The RBC method is found to provide the best artifact reduction and preservation of image features using both visual and quantitative assessment. The analysis and artifact-reduction methods are designed in context of FBP reconstruction motivated by computational efficiency practical for large, real synchrotron data. While a specific variable- truncation case is considered, the proposed methods can be applied to general data cut-offs arising in different in situ x-ray tomography experiments.}, language = {en} } @inproceedings{BorgJorgensenFrikeletal., author = {Borg, Leise and J{\o}rgensen, Jakob Sauer and Frikel, J{\"u}rgen and Quinto, Eric Todd and Sporring, Jon}, title = {Reducing artifacts from varying projection truncations}, series = {3rd International Conference on Tomography of Materials and Structures, Lund, Sweden, 26-30 June 2017, ICTMS2017-65-1}, booktitle = {3rd International Conference on Tomography of Materials and Structures, Lund, Sweden, 26-30 June 2017, ICTMS2017-65-1}, abstract = {We study samples with full and partial occlusion causing streak artifacts, and propose two mod-ifications of filtered backprojection for artifact removal. Data is obtained by the SPring-8 synchrotron using a monochromatic parallel-beam scan [1]. Thresholding in the sinogram segments the metal, resulting in edges on which we apply 1) a smooth transition, or 2) a Dirichlet boundary condition.}, language = {en} }